Camshaft phasing mechanisms allow selective adjustment of valve timing for internal combustion engines by selectively advancing or retarding the positions at least some of the lobes on a camshaft, thereby allowing associated valve movements to occur either earlier or later in the combustion cycle. For example, engines may operate more efficiently or effectively during one set of operating conditions when the valve timing is advanced, i.e., such that a valve(s) movement occurs earlier during the combustion cycle. Additionally, it may be desirable during a second set of operating conditions to retard the valve timing, i.e., such that a valve(s) movement occurs later during the combustion cycle. Adjusting the relative positions of at least some of the lobes on a camshaft allows internal combustion engines to operate with improved fuel economy, torque, and emissions.
Camshaft phasing mechanisms typically may be selectively positioned between at least two different settings ranging from fully advanced to fully retarded positions. Some systems employ oil pressure resulting from engine operation to selectively actuate phasing mechanisms during engine operation. These systems rely upon oil pressure to maintain a desired cam position and the resulting timing of a given valve. As a result, when engine operation ceases and oil pressure is lost the cam phasing mechanism may default to a position at either extreme of the phasing range, i.e., a fully advanced or fully retarded position. When the engine is subsequently started again, the valve will necessarily be in the default position for at least a short period of time, e.g., until engine oil pressure rises sufficiently to allow use of the phasing system. However, this may result in non-ideal engine start conditions. More specifically, at engine startup and especially during a cold temperature startup, it may be more desirable to have the camshaft positioned at a position in between the fully advanced and fully retarded position. There have been subsequent attempts at achieving an intermediate position, however proposed systems are complex and there is still a need for a cam phasing system that offers a sufficient degree of control over cam phasing when oil pressure may not be available, e.g., during engine startup conditions. Moreover, camshafts used in adjustable valve phasing systems that are hydraulically actuated may require one or more fluid passages in order to provide hydraulic pressure to selectively advance or retard timing of a cam lobe. Known examples of such camshafts rely upon fluid passages that are machined in the shaft or another part of the cam phaser. Forming such passages in these components can be costly and time-consuming.
Accordingly, there is a need for a camshaft assembly for a variable valve timing system that addresses the above problems.